With the spread of mobile devices in recent years, secondary batteries are being widely used. Among them, lithium ion secondary batteries, which are characterized by high charging/discharging voltage and large charging/discharging capacity, have attracted attention.
Conventionally, as positive electrode active material in a 4 V class high energy lithium ion secondary battery, spinel type structure LiMn2O4, layered rock salt type structure LiCoO2, LiCo1-xNixO2, LiNiO2, and the like are generally known. Among these, LiCoO2 excels in that it has a high voltage and high capacity, but the supply amount of cobalt material is small, leading to increased manufacturing cost, and there is also a problem from the viewpoint of environmental safety of waste batteries after use.
On the other hand, research into spinel structure lithium manganese oxide (basic composition: LiMn2O4) is thriving, because a large supply amount can suppress increases in cost, and manganese having good environmental suitability is used. Further, in a layered rock salt type structure of positive electrode active material, diffusion paths of Li are two-dimensional, whereas in a spinel structure of positive electrode active material, diffusion paths of Li are three-dimensional, which is of interest in positive electrode active material for secondary batteries, particularly for vehicular applications and stationary applications.
Here, when high crystallinity is developed in order to obtain high battery performance, obtained lithium manganese oxide particles have an octahedral structure that is the idiomorphic shape of a cubic spinel structure, and dissolution of Mn is likely to occur. Further, in a secondary battery using such a positive electrode active material, problems occur such as inferior storage properties at high temperatures.
Varied research and development (Patent Literature 1-4) has been undertaken to solve the problems of non-aqueous electrolyte secondary batteries using such lithium manganese oxide spinel structure positive electrode active material.